1. Field of the Invention
The present invention relates to a recording apparatus and a temperature detecting method therefor, and more particularly to an ink jet recording apparatus and a temperature detecting method therefor.
2. Related Background Art
Ink jet recording apparatus has recently become popular as the inexpensive recording apparatus capable of achieving high-quality recording and color recording. The recent trend employs an interchangeable recording cartridge, integrating an ink tank for storing the recording ink and a recording head for converting the electric signal from the main body of the recording apparatus into thermal energy thereby discharging the ink.
Such recording cartridge can reduce the cost by shortening the ink flow path from the ink tank to the recording head and can also reduce the amount of ink consumption at the ink discharge recovery operation by suction. Besides, by providing the ink tank with the ink of an amount matching the service life of the recording head, the replacement of the recording cartridge provides the advantage of effecting the ink supply and the maintenance of the recording head at the same time. Furthermore there is also provided a recording apparatus in which a color recording cartridge and a monochromatic recording cartridge are interchangeably used according to the purpose of the user.
In such ink jet recording apparatus employing the recording head which utilizes thermal energy for ink discharge, a known requirement for attaining high image quality is to control the electrical signal to be supplied to the recording head, according to the temperature thereof. This is because, if the electrical signal is constant, the amount of ink discharge varies depending on the temperature of the recording head, thus resulting in temperature-dependent unevenness in image density. The temperature of the recording head may be known for example by the use of a temperature sensor, or by estimation from the recorded data.
It has also been conducted to control the temperature of the recording head, by providing the recording head with a heat generating member for heating and driving said heat generating member according to the temperature of the recording head. A closed-loop temperature control is also known for example by providing the recording head with a temperature sensor. In case of so-called serial recording apparatus in which the recording is effected by the movement of the recording head, if a temperature sensor is employed as mentioned above, there is inevitably employed a flexible cable for sending the output voltage of said temperature sensor to an A/D converter in the main body of the recording apparatus, and there is required a countermeasure for the noises resulting from such flexible cable.
Instead of the temperature sensor provided on the recording head, there may also be employed a highly precise temperature sensor, such as a thermistor, provided on the control circuit board in the main body of the recording apparatus. Such configuration is to detect the ambient temperature of the recording head and to estimate the temperature of the recording head by calculation from the variation in said ambient temperature, in consideration also of the electric energy supplied to the recording head, the energy released by ink discharge and the energy dissipated to the external atmosphere. In such configuration, the thermistor mounted on the control circuit board of the main body of the recording apparatus is subjected to the influence of heat-generating components present on said board. Consequently the ambient temperature is obtained by estimating the temperature rise of the circuit board through the control of on/off time of such heat-generating components and subtracting the temperature rise resulting from the influence of such heat-generating components from the temperature indicated by the thermistor.
However, during the use of the recording apparatus, some of such heat-generating components present on the control circuit board are continuously activated while others are activated only during certain control operations, and the above-mentioned on/off time control without distinction in such activation modes may result in an error depending on the timing of thermistor reading. On the other hand, estimation of the temperature rise for example from the motor driving conditions, in order to avoid the abovementioned error, will require considerably complex calculations because the motor driving is complicated.
Also the above-mentioned temperature detection by the temperature sensor or by estimation from the recording data results in difficulties in case the recording cartridge is replaced to a new one after the recording head is heated by the execution of recording operation or in case the power supply of the apparatus is interrupted in the course of a recording operation.
For example the temperature estimation from the recording data requires history of the past recording operation, but such history becomes no longer usable in case the recording head is replaced as explained above.
Also in recent years, the number of recording elements has been increased for improving the recording throughput, since such increase in the number of recording element increases the number of pixels recordable in a single recording scan motion. On the other hand, such increase in the number of recording elements also leads to an increase in the temperature rise of the recording head, because of the increased heat generation of the recording elements. Also in case the throughput is improved by an increase in the driving frequency for the recording elements, there will result an increased temperature rise in the recording head because of an increase in the heat accumulation per unit time. Stated differently, an increased work rate given to the chip of the recording head increases the temperature rise in said chip. An excessively high temperature thus encountered in the chip of the recording head may lead to difficulties such as deformation of the constituent parts of the recording head.
For this reason the chip of the recording head is equipped with a temperature sensor, for constantly monitoring the temperature of said chip, and there is provided a protective sequence for suspending the activation of the recording elements in case a dangerous temperature is reached.
In general, the temperature sensor is composed of a diode sensor formed on a same silicon chip as that of the ink discharge heaters, because such sensor formed by film forming technologies is inexpensive and also because such sensor, formed on the silicon substrate of high thermal conductivity, is excellent in response. However, in the temperature-voltage relationship of such sensor, it is very difficult, within the manufacturing fluctuation, to retain zero-cutoff (offset) of said relationship within a practically acceptable tolerance, though the slope of said relationship can be well controlled. For this reason, the following process is adopted for calibrating the above-mentioned offset. In this process there are memorized a temperature (Tdef) corresponding to the voltage when the recording head is not heated and is equal to the room temperature, and a room temperature (Tr) obtained by the thermistor of the main body of the recording apparatus. For a temperature Tdi corresponding to the voltage of the head diode sensor at a certain state, the head temperature Th in said state can be given by: EQU Th=Tdi+Tadj EQU Tadj=Tr-Tdef.
Tadj mentioned above corresponds to the offset value of the head diode sensor.
However, if the recording head is replaced after it is heated, or if the user repeats the on/off operation of the power supply of the main body of the recording apparatus, the head reference temperature Tdef, indicated by the diode sensor, becomes erroneously set as the recording head temperature becomes higher than the room temperature. On the other hand, if a waiting period is programmed until the initial temperature is reached, the recording operation may not be conducted with the optimum conditions immediately after the mounting of the recording head.